Process and apparatus for the preparation of pulverulent metal oxides for ceramic compositions

ABSTRACT

The invention relates to a process and an apparatus for converting metal nitrate or mixed metal nitrate solutions into the corresponding metal oxides in a microwave field. The invention consists in heating, while the feed stream is being metered in, in such a manner that the solvent always evaporates within seconds and the decomposition product is obtained as a porous, sponge-like, purely oxidic material, which can be ground easily to give a powder having particle sizes of 0.5 to about 10 μm. The controlled metering in of the nitrate solution simultaneously makes it possible to control the reaction temperature below the sintering temperature of the powder. The reaction of the nitrate starting solution can take place continuously/batchwise in a reaction flask or continuously in a rotating reaction pipe, which is preferably charged with grinding balls, which possibly exert a reaction-activating effect similar to that exerted very particularly by the metal oxide sponge formed during the reaction.

BACKGROUND OF THE INVENTION

The invention relates to a process for the preparation of pulverulentmetal oxides for ceramic compositions from solutions of thecorresponding metal nitrates by evaporation of the solvent (water ordilute nitric acid) and thermal decomposition of the metal nitrate or ofmixtures of metal nitrates by means of microwaves. Furthermore, theinvention relates to an apparatus for carrying out the process.

The main purpose of the invention is to prepare metal oxide powders, inparticular to prepare pulverulent mixtures of oxides of various metals,for further processing to give ceramic compositions for, for example,varistors, piezoelectrics, ceramic superconductors, soft/hard ferrites,capacitors, microwave filters, ceramic sensors, such as oxygenpartial-pressure sensors, and the like.

As is known, the performance characteristics of the ceramics mentioneddepend not only on the homogeneity of the ceramic structure but also, inthe case of mixed oxide ceramics, very significantly on the homogeneityof the metal oxide mixture for the preparation of the ceramic. This iswhy the use of highly homogeneous, very fine and sinter-active powdersor powder mixtures is indispensable for preparing ceramic structuralcomponents of the type mentioned.

A plurality of processes for preparing ceramic mixed oxide powders isknown, the most important of which will be briefly outlined below.

In the so-called mixed oxide process, dry metal oxides and/or carbonatesare mixed in the desired ratio of the composition of the ceramic to beprepared, and, if desired, ground and then calcined in a crucible or ina rotary kiln at temperatures below the sintering temperature of thedesired ceramic. In order to improve the sintering activity of thepowders obtainable by the mixed oxide process, a vigorous grindingprocess is often required. During this process, material rubbed off fromthe grinding bodies reaches the powder material, which may result in adeterioration of the mechanical and electrical properties of theceramics obtained from the powders. In order to improve the homogeneityof the powder grain and powder mixture, it is in some cases necessary torepeat the grinding and calcining process several times. Nevertheless,the desired high-performance properties of the oxidic powders can beachieved only in a few cases.

More recently, the attempt has been made to counter the basicdisadvantages of the preparation of ceramic mixed oxide powders frompulverulent substances by preparing the oxide powders from solutions orsuspensions, for which the reaction spraying process, coprecipitation,the sol/gel process and the preparation of oxides or mixed oxides in amicrowave field are used. The initial forms of the substrates are herepreferably metal nitrates and metal alkoxides.

In the reaction spraying process, the mixture of the dissolvedcomponents is sprayed into a hot reaction space, in which the solutiondroplets are dried and converted to oxide particles while still in theair. Since delivery of energy to the droplets or particles only takesplace slowly and from the surface, hollow spheres are usually formed inthe spraying process (E. Ivers-Tiffee, K. Seitz "Characterization ofVaristortype, Raw Materials Prepared by the Evaporative Decomposition ofSolution Technique", American Ceramic Bulletin 66 [9], pages 1384-1388(1987)). When a ceramic is prepared from a powder comprising hollowspheres, some of these hollow spaces are exposed after sintering uponinvestigation of the structure. Ceramics with pores of this type havesignificantly lower strength compared with tight-burned ceramics.

In the coprecipitation process, the hydroxides of the initiallyintroduced metal salts are precipitated from a mixed salt solution byincreasing the pH, usually by adding ammonia, and the hydroxide mixturesare processed further to give the mixed oxide powder. The difficultiesof the process are the complicated process control (after extensiveempirical preliminary experiments) in order to achieve simultaneousprecipitation of the hydroxides of the different metal cations. In manycases, additional measures must be taken for the coprecipitationprocess, in order to obtain a sufficiently homogeneous composition ofthe hydroxide mixture precipitate, for example the addition of certainanions.

In contrast, the sol/gel process is an elegant and, in terms of thedesired product properties, satisfactory method. In this process,mixtures of metal alcoholates are preferably slowly hydrolysed and thensubjected to a polycondensation process which converts the hydrolysateinto a gel, which is then calcined to give the metal oxide powder. Theadvantages of the sol/gel process are, if the process of alcoholatehydrolysis and polycondensation is carried out with appropriate care,the homogeneous composition of the gel and the small particle size andsintering activity of the metal oxide powders obtainable from the gel.However, a particular disadvantage are the high production costs,determined in particular by the raw materials cost, of metal oxidepowders obtained in this manner. An operative disadvantage worthmentioning is the low solids content of the starting solutions and thegels as a result of the complex organic molecular radicals.

Finally, the preparation of metal oxides or metal oxides or metal oxidemixtures from the corresponding metal nitrates by thermal decompositionby means of microwaves, which is also the starting point for the presentinvention, is known, DE 32 32 867, J84 009 487, J59 079 179, DE 33 46253, J55 104 926, J56 145 104. However, the known processes are mostlylimited essentially to the one-component system manganese (for theproduction of dry-cell anodes), to the preparation of mixed oxides forglass- or ceramic-reinforced composites or structural components and tothe preparation of mixed oxides of uranium and thorium (for theproduction of nuclear fuel elements), for which one example each isgiven, but the first two of which do not start from dissolved but fromsolid metal nitrate.

According to Japanese Patent No. 067,946, solid manganese nitrate (150g) is subjected to pulsing microwave irradiation (2.45 GHz, 600 W) for12 minutes (in each case, 15 s of irradiation time with pauses inbetweenof 10 s) and, after a pause of 25 minutes, again subjected to thispulsing microwave irradiation for 2 minutes. For further processing, themanganese dioxide obtained is finely ground in a ball mill.

According to German Offenlegungsschrift 36 11 141, which relates to amixture for preparing fracture-resistant, fiber-reinforced ceramicmaterial by microwave heating, a fiber-reinforced fracture-resistantceramic material is obtained by the following principle: inorganicfibers (SiC; Si₃ N₄), oxidic materials (glass; Al₂ O₃ ; ZrO₂), a solidnitrate (NaNO₃ ; Zr(NO₃)₄) and a coupling agent (preferably glycerol)are mixed and subjected in a heat-resistant reaction vessel (diameter:2.5 cm, height: 2cm) to microwave radiation (2.45 GHz) for 1 to 2 h,during which temperatures of around 1000° C. are reached. The meltproduced leads to a fractureresistant ceramic article. The long durationof the microwave irradiation serves, inter alia, to obtain an orderedspatial orientation of the reinforcing fibers.

According to U.S. Pat. No. 4,444,723, the oxidic materials are producedfrom solutions of plutonium nitrate or uranyl nitrate or mixtures ofthese nitrates in a microwave field by heating the solution to 100° to120° C., evaporating the solvent until a moist nitrate cake is obtained,heating the nitrate cake to 350° to 400° C. for converting the metalnitrate into the metal oxide and then calcining the metal oxide at 700°to 800° C. The reaction set-up essentially comprises a circulatingcontinuous belt carrying a plurality of open and rotating reactiondishes which pass through at least three microwave chambers arranged oneafter the other, in which the nitrate solution is first evaporated, thenitrate cake obtained, after a steep increase in temperature withconstant temperature control and adjustment, is then denitrated to givethe metal oxide and finally the metal oxide obtained is calcined to givethe process product upon passing through a plurality of microwavechambers, likewise with constant temperature measurement and adjustment.The temperature of the irradiated material is adjusted by mechanicalvertical movements of the reaction dishes moving horizontally androtating on the continuous belt in the microwave field.

Although the conversion of metal nitrates to metal oxides in themicrowave field is in principle known, the known processes cannot beutilised for preparing, in particular, mixed oxide powders forhigh-performance ceramics. Some of the reasons are as follows:

When starting from dry mixtures of the nitrates, fundamentaldisadvantages similar to those of the mixed oxide process describedabove have to be accepted. A further disadvantage, which can be derivedfrom U.S. Pat. No. 4,444,723, is that nitrate materials initiallyintroduced in dry form or obtained by evaporating a nitrate solution,after reaching a higher temperature and thus enhanced couplingproperties with microwaves, have the tendency often to heat up rapidlyand with glowing in an uncontrollable manner, which may lead to rapidsintering of the material. Owing to the regular inhomogeneities of themicrowave field, possibly also owing to other causes, the glowing isoften only limited locally. If the reaction is interrupted by switchingoff the microwave field and then continued, the same areas glow again,while the portions of the reaction material next to them take adifferent course of the reaction. As a result, the final products do notconform to the requirements of compositions or powders for preparinghigh-performance ceramics with respect to particle size homogeneity (ifsintering to give more compact units has not already taken place) orwith respect to a homogeneous distribution of the elements. Furthermore,the process products do not turn out to be nitrate-free with certainty

If, according to the fundamental teaching of U.S. Pat. No. 4,444,723,the starting materials are fairly large batches of mixed nitratesolutions, the comparatively slow evaporation of the mixed nitratesolution is bound to lead to partial separation of the components in theprecipitate and to a mixed oxide product of accordingly inhomogeneouscomposition, due to the different solution properties of the individualmetal nitrates. In the case of fairly large volumes of nitrate materialsto be converted to the mixed oxide by means of microwaves, other factorscausing separation are the different reaction temperatures and rates ofdecomposition which are site-dependent (on the surface and in thedepth). In the case of partial sintering or even melting, further mixingand particle inhomogeneities occur.

It may also be mentioned that the reaction of solid metal nitratesencounters difficulties whenever the nitrate form is incapable ofcoupling with the incoming microwave irradiation at low temperatures,for example room temperature. An example of such a metal oxide isalumina. If nevertheless it is desired to decompose such a metal nitratethermally by means of microwaves, the nitrate or mixed nitrate has to beheated to the coupling temperature either by means of a thermal energysource or a coupling agent (say glycerol, according to the teaching ofGerman Offenlegungsschrift 36 11 141, or ammonium nitrate whichdecomposes without leaving a residue) has to be added. However, whenstarting from aqueous nitrate solutions, the situation where the metalnitrate or nitrate mixture to be reacted in the dry form by means ofmicrowave irradiation has not yet reached the temperature of responsefor coupling with the microwaves can be avoided, except for a few cases.The reason for this is that water shows good coupling properties withmicrowaves, as a result of which an aqueous nitrate solution heatsrapidly, the solvent can be readily evaporated and the metal nitrateobtained in crystalline form after evaporation of the solvent is presentat temperatures of more than 100° C., at which temperature most metalnitrates show good interaction with the microwaves.

SUMMARY OF THE INVENTION

Compared with the prior art for preparing mixed oxide powders forceramic compositions, in particular high-performance ceramiccompositions, which is essentially limited to the mixed oxide process,and utilizing the decomposition technique of mixed metal nitrates togive mixed metal oxides, according to U.S. Pat. No. 4,444,723, whichstarts from dissolved metal nitrates, an object of the invention is toprovide a universal process and apparatuses adapted thereto, by means ofwhich metal oxide powders, but in particular mixed metal oxide powders,useful for high-performance ceramics having extremely homogeneous ratiosof the particle size and distribution of the elements in the mixed oxidepowder, can be obtained in an effective manner at low cost and fordirect further processing to give the desired ceramic.

Starting with the essential features of the process according to U.S.Pat. No. 4,444,723, i.e. evaporation of the solvent of (mixed) nitratesolutions and thermal decomposition of the solid nitrate thus formed togive the oxidic form by means of microwaves, the object is achieved by aprocess for the preparation of pulverulent metal oxides for ceramiccompositions from the corresponding metal nitrates dissolved in water orin dilute nitric acid, by evaporation of the solvent and thermaldecomposition of the metal nitrate by means of microwaves, characterizedin that metal nitrate solution is initially introduced into a reactionvessel such that when the solvent is evaporated by means of microwaveirradiation until nitrous gases are formed and, while microwaveirradiation is continued, further metal nitrate solution is addedcontinuously onto the porous metal oxide containing sponge in a mannersuch that the solvent rapidly evaporates and the newly added metalnitrate is immediately converted to the metal oxide. The inventionfurther provides an apparatus for continuous operation of the process,comprising a rotating reaction pipe running through a microwaveresonator and made of a material permeable to microwaves and equippedwith a feed inlet for the oxidic reaction product.

The invention is described below in more detail first in its generalfeatures and then by way of example.

A central idea of the invention is the concept that the disadvantageswhich must always be accepted when fairly large volumes of, inparticular, mixed metal nitrate solutions are converted in aconventional manner into the oxidic metal oxides by means of microwaves,i.e. on the one hand the possible incomplete conversion of the nitratesinto the oxides and on the other hand the appearance of oxidic productsof inhomogeneous composition with respect to the distribution of theelements, can be avoided by starting the reaction not with the entirebatch but using the (mixed) nitrate solution to be convertedcontinuously in small portions so that the solvent portion of thesolution added per time unit is rapidly evaporated and the solid metalnitrate which in each case is formed in small amounts per time unit isdecomposed to give the metal oxide in a rapid reaction. If the reactionproceeds in the manner described, separation processes as a result ofdifferent metal nitrate solubilities during the, as it were, spontaneousevaporation of the starting solution and further inhomogenizing effectsduring the thermal decomposition of the solid mixed nitrate to give theoxidic form are prevented. Since in the continuous "immediateconversion" of the nitrate solution to the corresponding oxide the freshsolution introduced into the reaction process essentially only reachesthe surface of the previously formed metal oxide composition each time,the conversion of the nitrate to the oxide takes place virtually as asurface reaction (on the previously formed oxide composition) andtherefore leads to the ensured complete conversion of the nitrate to theoxide An important feature is that in this quasi-continuous reactioncontrol a partial sintering of the metal oxide composition by local,uncontrolled superheating, which may produce glowing, can be safelyprevented.

If the reaction is carried out in such a manner that first a smallamount of nitrate solution is initially introduced into a storagevessel, for example made of Duran® glass, (Schott, Mainz, Germany) andthis solution is evaporated to dryness by means of microwaveirradiation, which results in a nitrate cake of temperatures of around100° to 120° C., and irradiation is then continued until nitrous gaseshave been formed, the nitrate composition is converted in many casesreadily into a finely porous oxide or nitrate/oxide sponge, whichcontinues to form during the subsequent continuous addition of furthernitrate solution and, at the end of addition of nitrate solution and itsconversion, turns out to be a purely oxidic, nitrate-free product. Theoxide sponge removed after cooling can be readily ground, for example inan agate mortar, to give a fine powder, which can be directlyplasticized and compacted. In the case of mixed oxide powders preparedin this manner, the material turns out to be extremely homogeneous; thedistribution of elements is completely uniform over the entire product(no variation of the element mixing ratios as a function, for example,of the depth of the sample removed from the reaction material), and theoxide or mixed oxide crystallites obtained regularly have particle sizesin the range from 1 to 10 μm. However, if any solid bridges have beenformed between the particles of the final product, they can be destroyedby crushing, if desired by gently grinding, the process product.

The sponge-like porous consistency of the product formed has anexcellent effect of the conversion of further starting material on thesponge substance. If this desired porous, sponge-like consistency of themetal oxide formed should not form or should form incompletely withcertain metal nitrates, the addition of an organic acid soluble in thestarting nitrate solution and precipitating in the form of crystals uponevaporation of the solvent, for example tartaric acid, ensures that theoxidic process product formed is porous and sponge-like. Furthermore,such an addition has an accelerating effect on the conversion.Preferably, the organic acid is added in an amount of 1-30% b.w., morepreferably, 5-10% b.w., of the total solution.

The favorable effect mentioned of the metal oxide sponge on the furtherconversion of metal nitrate solution to the metal oxide is based on athermal effect and probably also on a surface effect of the porous,sponge-like product composition.

The thermal effect can be explained by the fact that the freshly addednitrate solution, aided by the heat release of the solid material, canevaporate on the heated sponge mass almost spontaneously--which wouldnot be possible to achieve by pure addition of microwave energy--andthat the solid nitrate formed in the meantime is immediately present ata temperature which, in the usual case, is above the minimum couplingtemperature with the microwaves. The high overall rate of conversionalso explains the uniformity and especially the small particle size ofthe crystallites of the oxidic process product.

The process of the invention makes it possible, in a very simple manner,to control the process procedure without interruption or change inenergy of the microwave field acting on the reaction mass by controllingthe adjustment and stabilization of the substance temperatures in thereaction vessel via the feed stream of fresh metal nitrate solution. Abalanced metering in of the fresh solution also avoids glowing of thereaction or product composition. Furthermore, it was found that the riskof glowing of the microwave-irradiated substance after formation of thesponge-like product is very small anyway, probably because the microwaveenergy absorbed over the very large surface of the sponge substance iseasily removed by thermal irradiation or convection. The selection ofmicrowave radiation is such that the frequency, wattage, and time issufficient to decompose the bulk of the nitrate to oxide. Thedetermination thereof may be made with routine experimentation.Conventional microwave equipment is sufficient to effect the process.

In the reaction in the reaction flask using microwaves of 2.45 GHz andan output of the microwave generator of 1000 watt, it is, for example,possible to convert 1000 g of mixed oxide powder to the desiredsponge-like end product within about 30 to 90 minutes in the followingmanner: about 100 ml of nitrate solution are initially introduced intothe flask and irradiated with microwaves over a period of 5 to 20minutes until nitrous gases are formed and the resulting solidcomposition has a sponge-like consistency; while maintaining themicrowave irradiation, the remaining supply of nitrate solution is thenrun in in such an amount that the solvent always evaporates in a fewseconds. The temperature of the dry mass formed is here in the rangefrom 100° to 500° C. and can be controlled, as mentioned, by the flowintensity of the feed solution. The nitrous gases formed in the reactioncan be trapped or sucked off and, if desired, neutralized by generallyknown measures.

However, the reaction also works if smaller amounts of nitrate solutionare initially introduced or, with the microwave field in place, only ahigher first feed dose of nitrate solution are initially introduced or,with the microwave field in place, only a higher first feed dose ofnitrate solution is chosen. One of ordinary skill in the art candetermine operating parameters with only routine experimentation, as thespecific amounts of starting materials are not critical within a widerange, and the starting solution concentration affects the time taken toproduce the sponge but not the ability to do so.

Instead of carrying out the process according to the invention in aquasi batchwise/continuous process in the storage vessel, it can also becarried out continuously by designing the reaction space in the mannerof a rotary kiln and adding the nitrate solution at one end thereof andremoving the reaction product (metal oxide or mixed metal oxide) at theother end thereof. In this design, the reaction tube can be made ofDuran or quartz glass and is disposed in a microwave resonator spacewhich in turn is surrounded by a radiation-shielding casing.

The preparation of the metal oxides in a rotary tube may be conducted inthe presence of grinding bodies, thereby advantageously combining theprocess with a continuous gentle grinding process for immediatepulverization of the metal oxide sponge formed otherwise and fordestroying any solid bridges formed in the reaction product, in whichthe grinding bodies furthermore have an energy-transferring functionanalogous to that of the metal oxide sponge. These grinding bodies,preferably in spherical form, can be, for example, alumina, which,although having poor coupling properties with microwaves, is heated tothe general reaction temperatures in the course of the thermaldecomposition of the metal nitrate or the mixed metal nitrate. On theother hand, in order to accelerate the evaporation and conversionprocesses, it is also possible to use grinding bodies consisting of amaterial which readily couples with microwaves even at low temperatures,and thus heats due to the interaction with the incoming radiation. Anexample of such a material is silicon carbide. This results in thepractical advantage of using alumina grinding bodies for nitrates whichhave good coupling properties and of silicon carbide grinding bodies fornitrates which have poor coupling properties.

The technical idea of using grinding bodies made of a material whichitself either has good coupling properties with microwaves and is heatedin the process or which has no to poor coupling properties withmicrowaves and is not substantially heated directly by the incomingenergy can be put to use to particular further advantage by initiallyintroducing into the rotating tube a reaction bed comprising a mixtureof grinding bodies of good coupling properties and grinding bodies ofpoor coupling properties. The obtainable technical effect is that,depending on the coupling properties of the different materials of thegrinding bodies, the mixing ratio of the materials of the grindingbodies and the incoming microwave energy density, the reaction bed canbe brought to a predeterminable starting temperature level before thebeginning of the reaction by irradiation, after which the nitratesolution to be converted is then added continuously. Moreover, in thecase of a weight ratio of grinding body mixture and reaction materialmatched to one another a temperature level of the mixture comprisingreaction bed and product formed can be set to a predeterminable value,i.e. the general reaction temperature can be adjusted and ,kept constantwith particular ease during the course of the reaction, in particularafter the equilibrium between starting nitrate solution fed in andoxidic reaction product discharged has been reached. This excellentlyensures an optimal temperature control matched to the particularstarting solution of the metal nitrate or mixed metal nitrate. Insteadof mixing the reaction bed comprising the grinding bodies whichthemselves either consist of material having good or poor couplingproperties with microwaves (for example silicon carbide grinding bodiesand alumina grinding bodies), it is also possible to use grinding bodieswhich themselves already comprise a mixture of materials having good andpoor coupling properties. The use of grinding bodies of uniform(coupling or non-coupling) composition is preferable in those caseswhere it is of interest to readjust the temperature level of thereaction bed mixture in a convenient manner via the mixing ratio of thegrinding bodies having good and poor coupling properties withmicrowaves, which can be effected by additionally metering in grindingbodies of the suitable thermally active species to the reaction bedmixture or by removing in grinding bodies of the other species having anopposite thermal effect from the reaction bed mixture. If the variousmaterials of the grinding body can already easily be distinguishedoptically, the reaction bed mixture can be separated manually withoutmuch trouble; a separation can furthermore be carried out quickly byscreening, if grinding body species of different specific diameters arepresent.

DESCRIPTION OF THE DRAWINGS

The invention is described in more detail below by way of exemplaryembodiments, for which .first one example each for a reaction set-up forcarrying out the process in a storage vessel and for carrying it out ina reaction rotating tube are presented in one schematic drawing each.

FIG. 1 shows a microwave reaction plant for preparing oxidic powdersfrom aqueous nitrate solutions in a quasi batchwise/continuous processand

FIG. 2 shows a microwave rotary kiln for preparing oxidic powders fromaqueous nitrate solutions in a continuous process.

FIG. 3 is a graph which shows the performance of the varistor ceramic ofthe present invention compared with the performance of a conventionalvaristor ceramic.

DETAILED DESCRIPTION

The apparatus according to FIG. 1 includes three-neck flask 1 as areaction vessel connected to a storage vessel 2 for a metal nitratesolution 3 to be converted in the reaction vessel. The nitrate solution3 is added to the reaction vessel 1 through a feed inlet 5 in ameterable feed stream via a control valve 4. In a modified arrangement(not shown), the nitrate solution 3 can also be injected or sprayed intothe reaction vessel 1. The reaction vessel 1 is enclosed in a chamber 6of a microwave resonator. Through the upper shielding of the chamber 6,the feed inlet 5, a purge gas feedline 7 and a gas discharge line 8 (ifappropriate connected to a downstream suction pump--not shown) pass andare connected to the necks of the three-neck flask 1. The output of themicrowave generator (not shown) and its operating time is adjusted bymeans of adjusting devices 9 and 10. Reference numeral 11 designates theoxidic reaction product.

The apparatus according to FIG. 2, i.e. a microwave reaction plant forcarrying out the process according to the invention continuously,comprises as an essential component a rotation reaction tube 12, whichcan be manufactured, for example, from Duran or quartz glass, into whicha feed inlet device 13 for adding the nitrate or mixed nitrate solution3 to be converted opens. At the opposite end of the tube 12, a dischargedevice 14 is positioned for discharging and collecting the oxidicprocess product. The reaction tube 12 runs through a microwave resonator15, is supported at each end on a bearing 16 and can be made to rotateby means of a drive unit 17, the power transmission taking place, forexample, by means of a driving belt 18. At least one of the bearings 16is height-adjustable, in order to give the reaction tube 12 anadjustable inclination of, preferably 5° to 15°. For the shieldingexterior from the microwave radiation in the embodiment shown in FIG. 2,lateral metal shields 19 are provided as well as grids 20 positionedoutbound of the metal shields. The casing of the microwave resonator 15and the metal shields 19 are arranged directly in front and behind thereactor tube being connected conductively by means of a sliding contact21.

To completely shield the surroundings from micowave irradiation, it isrecommended to surround the entire reaction set-up with radiationcontaining casing 22 which, of course, includes closable sides (notshown) to the reaction apparatus. As explained above, the reaction tube12 can be charged with grinding bodies 23, which on the one handmaintain the oxidic reaction product formed in pulverulent form orcondition it into a pulverulent form when the reaction tube 12 isrotated, and on the other hand, exert an energy-transmitting functionanalogously to that of the metal oxide sponge 11.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing and in the following examples, all temperatures are setforth uncorrected in degrees Celsius and unless otherwise indicated, allparts and percentages are by weight.

The entire texts of all applications, patents and publications, citedabove and below, and of corresponding German application P 40 34 786.9are hereby incorporated by reference.

EXAMPLES Example 1

Conversion of a Zn--Sb--Bi--Mn--Cr mixed nitrate to the mixed oxide withaddition of tartaric acid (for preparing a varistor ceramic).

About 1000 g of varistor powder were prepared using an experimentalset-up described under FIG. 1 at a microwave frequency of 2.45 GHz and amicrowave output of 1000 watt in the following manner.

The starting solution contained the substances and amounts of substanceslisted below as an aqueous solution; the concentration of the solutionwas about 20% by weight, calculated as the oxide form of the dissolvedsubstances:

    ______________________________________                                        Zn(NO.sub.3).sub.2 × 6H.sub.2 O*.sup.)                                                        3158.3  g                                               Sb.sub.2 O.sub.3      32.2    g                                               Bi(NO.sub.3).sub.3 × 5H.sub.2 O                                                               160.9   g                                               Mn(NO.sub.3).sub.2 × 4H.sub.2 O                                                               13.9    g                                               Cr(NO.sub.3).sub.3 × 9H.sub.2 O                                                               44.3    g                                               Co(NO.sub.3).sub.2 × 6H.sub.2 O                                                               48.3    g                                               ______________________________________                                         *.sup.) dissolved in a mixture of 125 g of tartaric acid and 250 ml of        water.                                                                   

First 100 ml of this solution were initially introduced into thereaction flask 1 and evaporated to dryness over a period of about 15minutes with exposure to microwave radiation until nitrous gases formed.The temperature of the solid substance was about 300° C. The remainingsolution was then added dropwise to the porous product sponge formedover a period of about 50 minutes in such a manner that the solventimmediately evaporated in each case and the sponge-like structureenlarged. 5 minutes after the addition of nitrate solution was complete,the microwave field was switched off, the cooled porous sponge materialwas then removed and crushed in an agate mortar. The fine powderobtained could be plasticized, compacted and sintered at 1200° C. for 90minutes without problems to give a varistor ceramic.

The powder obtained substantially had a particle size of 2 μm (withdeviations in the range from 0.5 to 5 μm) and the analytical (ACTUAL)composition listed in Table 1, which is followed by the theoretical(NOMINAL) composition according to the reaction batch:

                  TABLE 1                                                         ______________________________________                                        Composition of the mixed oxide powder obtained according                      to Experiment 1 in mol %                                                              ZnO  Sb.sub.2 O.sub.3                                                                      Bi.sub.2 O.sub.3                                                                      MnO.sub.2                                                                           Cr.sub.2 O.sub.3                                                                    Co.sub.3 O.sub.4                     ______________________________________                                        ACTUAL    86.31  3.20    7.65  0.40  0.87  1.42                               NOMINAL   86.39  3.22    7.73  0.43  0.88  1.33                               ______________________________________                                    

Thermogravimetric analysis (TGA) showed that complete conversion to theoxides had taken place. The weight loss of the samples was about 1% byweight at 800° C. The weight loss took place continuously, starting atabout 50° C. onwards, which indicates moisture adsorbed on the surfaceand does not indicate subsequent conversion of nitrate which may nothave been converted completely to the oxide.

Chemical analysis of samples of the product which were taken fromdifferent areas (surface, center, depth) of the product sponge showedthat within the experimental error of the analytical procedure allsamples had the same composition of doping elements (Table 2).

                  TABLE 2                                                         ______________________________________                                        Analysis of various samples of a charge                                       The values were determined by x-ray fluorescence analysis                     (XRFA)                                                                                  Data in atom %                                                      Sample number                                                                             Bi       Co     Cr     Mn   Sb                                    ______________________________________                                        a           0.42     0.49   0.43   0.37 0.86                                  b           0.42     0.44   0.40   0.37 0.84                                  c           0.37     0.44   0.41   0.37 0.81                                  d           0.40     0.43   0.41   0.37 0.83                                  e           0.43     0.46   0.39   0.37 0.81                                  ______________________________________                                    

The performances of the varistor ceramic produced are shown in FIG. 3(curve A) in comparison with a conventional varistor ceramic producedfrom an oxide powder of the same composition obtained by the mixed oxideprocess (curve B).

As can be seen, the advantages of the varistor ceramic produced using azinc oxide powder prepared according to the invention are the formationof a steeper and sharper bend of the varistor characteristic and theappearance of smaller leakage currents at the point of use, which onlyappear at a higher voltage. The higher varistor qualities compared withthe prior art indicate that the zinc oxide varistors produced usingmixed oxide powders according to the invention are distinguished by ahigher concentration of active grain boundaries than the varistorsobtainable from oxide powders obtained in a conventional manner. Thematerial prepared according to the invention makes it possible toproduce thinner varistors than those produced conventionally and toconsume less material.

Example 2 Preparation of yttrium-stabilized zirconium dioxide powder

A nitric acid solution of zirconium oxycarbonate and yttrium oxide werereacted using an experimental set-up such as described in FIG. 1 at amicrowave frequency of 2.45 GHz and a microwave output of 1000 watt, inwhich the composition of the starting solution was about 90 mol % ofzirconium and about 10 mol % of yttrium and the concentration by weight,relative to the oxides, was that of a 20% solution.

First about 100 ml of this solution were initially introduced into thereaction flask 1 and heated in a microwave field until nitrous gaseswere formed. This resulted in the formation of a porous sponge material.Further starting solution was then metered in in such a manner that onthe one hand the amount of solution added in each case was convertedwithin seconds in the microwave field and on the other hand thetemperature was maintained at a moderate level. The reaction productobtained, on the whole a porous sponge material, could be crushed in anagate mortar to give an extremely fine powder whose crystal sizes werebetween 0.5 and 2 μm. The composition of the product is shown in Table3.

                  TABLE 3                                                         ______________________________________                                        Composition of the yttrium/zirconium oxide powder owder                       obtained by Example 2 in mol %.                                                             Zirconium                                                                             Yttrium                                                 ______________________________________                                        ACTUAL:         90.34     9.70                                                NOMINAL:        90.23     9.77                                                ______________________________________                                    

Dilatometer tests showed that the product material starts sintering at atemperature as low as about 700° to 800° C. The most extensive sinteringbehavior was shown by this material in the range between 1150° and 1250°C. At a temperature of about 1500° C., sintering is complete This showsthat an yttrium-stabilized zirconium dioxide prepared by the processaccording to the invention has a high sintering activity even atrelatively low temperatures compared with conventionally preparedpowders, which require sintering temperatures between 1700° and 1900° C.

Example 3 Preparation of yttrium-barium-copper oxide powder (for asuperconducting material to be produced therefrom)

A 20% solution (calculated as oxide content) of Y(NO₃)₂, Ba(NO₃)₂ andCu(NO₃)₂ which had a molar Y:Ba:Cu mixing ratio of 1:2:3 was reactedusing an experimental set-up such as described under FIG. 1 at amicrowave frequency of 2.45 GHz and a microwave output of 1000 watt.

In identical experimental batches, first amounts of 50 to 100 ml ofsolution were in each case initially introduced into the reaction flask1 and evaporated to dryness by means of microwave irradiation. Thereaction temperature here was about 400° C. After the formation ofinitial amounts of nitrous gases, which were sucked off, furthersolution was metered in until about 250 g of product had been obtained.After further irradiation with microwaves for about 15 minutes, a blackyttrium-barium-copper oxide powder was removed, which did not have to berecalcined. The material could be easily crushed in a mortar to give afine homogeneous powder.

The powder thus obtained was then compacted to give tablets andsintered. The superconducting property of this material could bedetected by dipping the tablets into liquid nitrogen and making themfloat above a ring magnet. Accordingly, the assumption may be made thatthe critical temperature is above the temperature of liquid nitrogen.

Example 4 Preparation of a varistor powder in a continuous microwavereaction process in a rotating pipe

A varistor powder was prepared in a reaction setup according to FIG. 2comprising a rotating pipe 12 made of Duran glass of 500 mm in lengthand an internal diameter of about 80 mm at a microwave frequency of 2.45GHz and a microwave output of 2×500 watt as follows:

500 to 700 ml of a starting solution prepared as in Example 1 wereinitially placed in the reaction pipe 12 whose rotational speed wasbetween 15 and 30 min⁻¹ and had been charged with about 800 g of aluminagrinding bodies (spheres), and concentrated with exposure to microwaveirradiation until nitrous gases were formed. The temperature of thesolid substance obtained was about 400° C. Further starting solution wasthen metered in continuously over a period of about 100 minutes in sucha manner that the solvent immediately evaporated each time. The drysubstance of the product increased, was obtained as a fine powder byvirtue of the rotating movement of the reaction pipe 12 and the grindingbodies 23 and continuously discharged at the end of the reaction pipe12. The purely oxidic pulverulent product obtained in this waypredominantly had particle sizes in the range from 1 to 15 μm, eventhough finer particles were also present. The powder obtained wasplasticised, compacted and sintered at 1150° C. without difficulties togive a varistor ceramic. The electrical values of the varistor ceramicwere those of the varistor ceramic according to Example 1.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. In a process for the preparation of pulverulentmetal oxides for ceramic compositions, from the corresponding metalnitrates dissolved in water or in dilute nitric acid, comprisingevaporation of the solvent and thermal decomposition of the metalnitrate by means of microwaves, the improvement wherein a quantity ofmetal nitrate solution is initially introduced into a reaction vesselsuch that when the solvent is evaporated by means of microwaveirradiation until nitrous gases are formed, a porous sponge containingmetal oxides is formed, and, while microwave irradiation is continued,further metal nitrate solution is added continuously onto the porousmetal oxide containing sponge in a manner such that the solvent rapidlyevaporates and the newly added metal nitrate is immediately converted tothe metal oxide.
 2. A process according to claim 1, wherein the reactiontemperature is controlled by adjusting the amount of metal nitratesolution added per unit of time.
 3. A process according to claim 1,wherein the nitrate solution contains a crystalline organic acid in asolvent.
 4. A process according to claim 3, wherein the crystallineorganic acid is tartaric acid.
 5. A process according to claim 1,wherein the reaction is carried out in a rotating reaction pipe.
 6. Aprocess according to claim 5, wherein grinding bodies are present in therotating pipe.
 7. A process according to claim 6, wherein the grindingbodies comprise a material which absorbs microwaves and is therebydirectly heated.
 8. A process according to claim 6, wherein the grindingbodies comprise a material which does not absorb a significant amount ofmicrowaves, and is not thereby directly heated.
 9. A process accordingto claim 6, comprising using a mixture of grinding bodies of which someabsorb and some do not absorb microwaves, whereby some are heated andsome are not heated by the microwaves, in the form of a reaction bedhaving a predeterminable reaction temperature level.
 10. A processaccording to claim 7, wherein silicon carbide is the material of thegrinding bodies absorbing the microwaves and alumina is the material ofthe grinding bodies not absorbing microwaves.